Abstract: A process for large-scale production of CdTe/CdS thin film solar cells, films of the cells being deposited, in sequence, on a transparent substrate, the sequence comprising the steps of: depositing a film of a transparent conductive oxide (TCO) on the substrate; depositing a film of CdS on the TCO film; depositing a film of CdTe on the CdS film; treating the CdTe film with CdCl2; depositing a back-contact film on the treated CdTe film. Treatment of the CdTe film with CdCl2 comprises the steps of: forming a layer of CdCl2 on the CdTe film by evaporation, while maintaining the substrate at room temperature; annealing the CdCl2 layer in a vacuum chamber at a temperature generally within a range of 380° C. and 420° C. and a pressure generally within a range of 300 mbar and 1000 mbar in an inert gas atmosphere; removing the inert gas from the chamber so as to produce a vacuum condition, while the substrate is kept at a temperature generally within a range of 350° C. and 420° C.